1. Field of the Invention
This invention relates to a lens position control device adapted for optical apparatuses such as video cameras, silver-salt cameras and interchangeable lenses.
2. Description of the Related Art
Among the photo-taking zoom lenses of the varied kinds generally employed for a video camera or the like, a "front-lens focus type" zoom lens is most popular. In the front-lens focus type zoom lens, a first lens group is used as a focus lens for adjusting focus; a second lens group is used as a variator lens for varying the power of the zoom lens; a third lens group is used as a compensator lens for keeping the image forming position of the zoom lens constant even when a power varying action is performed; and a fourth lens group is used as a relay lens for forming an image.
The positional relation between the variator lens and the compensator lens of the front-lens focus type zoom lens is preset and unvarying irrespectively of the position of the front lens, i.e., the focusing distance. In most cases, therefore, the variator lens and the compensator lens are interlocked by means of a mechanical part called a cam ring.
FIG. 7 of the accompanying drawings shows the arrangement of the front-lens focus type zoom lens which is generally employed. Referring to FIG. 7, the zoom lens consists of a focus lens 101 which is a first lens group; a variator lens 102; a compensator lens 103; and a relay lens 104. These lenses respectively have the above-stated functions. The illustration includes a fixed tube 105; a female helicoid 106; a front-lens tube 107; a relay holder 108; a relay tube 109; a diaphragm blade unit 110; an aperture meter 111; a zoom motor 112; a gear head part 113 for the zoom motor 112; a focus motor 114; a gear head part 115 for the focus motor 114; an output gear 116 of the zoom motor 112; an output gear 117 of the focus motor 114; a gear part 118 formed on the female helicoid 106 in one body with the latter; a zoom ring 119; a gear part 120 formed on the zoom ring 119 in one body with the latter; a projection 121 arranged to transmit the rotation of the zoom ring 119 to a cam ring 122; the cam ring 122; a cam slot 123 provided in the cam ring 122 for the variator lens 102; a cam slot 124 for the compensator lens 103; a variator-lens moving ring 125; a compensator-lens moving ring 126; a cam follower part 127 formed in one body with the variator-lens moving ring 125; a cam follower part 128 formed in one body with the compensator-lens moving ring 126; guide bars 129 and 130 of the moving rings 125 and 126; a focus-motor slip unit 131; and a zoom-motor slip unit 132.
FIG. 8 is an oblique view mainly showing the compensator lens part. In FIG. 8, the same reference numerals as those of FIG. 7 indicate the same parts.
The front-lens focus type zoom lens having the above-stated parts performs the following actions:
FOCUSING ACTION: The focus lens 101 is secured to the front-lens tube 107 by heat caulking or the like. The periphery of the front-lens tube 107 is fitted into the inner side of the female helicoid 106 without any play and is fixed in position by some adhesive or the like after adjustment of its position in the direction of an optical axis. The female helicoid 106 is screw-fitted by a helicoid screw on the fixed tube 105 in its rear part. The focus lens 101 is thus arranged to be movable in the direction of the optical axis by rotating the female helicoid 106. Further, the output gear 117 of the focus motor is interlocked with the gear part 118 of the rear end part of the female helicoid 106. Upon receipt of a driving instruction from an automatic focusing device or the like which is not shown, the focus motor 114 rotates. The rotation of the focus motor 114 is moderated by the gear head part 115 to cause the focus lens 101 to move via the slip unit 131. In the case of manual focusing, the female helicoid 106 is operated by the operator. In this case, the gear provided within the gear head part 115 is prevented from being damaged by the operation as the slip torque of the slip unit 131 is set at such a value that prevents such damage.
ZOOM ACTION: In the front-lens focus type zoom lens, the variator lens 102 and the compensator lens 103 must be interlocked to be constantly in a given positional relation. To maintain this positional relation, the cam ring 122 is provided with the cam slot 123 for the variator lens 102 and the cam slot 124 for the compensator lens 103. Two guide bars 129 and 130 which are arranged as shown in FIG. 8 are used for guiding the variator lens 102 and the compensator lens 103 in the direction of the optical axis. In the case of FIG. 8, a sleeve part which is in one body with the compensator-lens moving ring 126 is fitted on the bar 130. The bar 129 serves as a rotation stopper. The cam follower part 128 engages the cam slot 124. The variator lens 102 and the compensator lens 103 are thus arranged to move in association with each other when the cam ring 122 is rotated. The outer circumferential face of the cam ring 122 and the inner circumferential face of the fixed tube 105 are fittingly engaged in such a dimensional relation as to be turnable by a slight torque without rattling. The cam ring 122 is located on the inner side of the fixed tube 105. The cam ring 122 must be rotated in response to a rotating operation performed on the zoom ring 119 by the operator. For this purpose, the projection 121 is provided at the rear end of the zoom ring 119 for interlocking the zoom ring 119 with the cam ring 122.
Therefore, the fixed tube 105 is provided with a slot which extends over the turning range of the projection 121, covering the rotation angle of the zoom ring 119 between the telephoto and wide-angle end positions of the zoom ring 119. The zoom ring 119 and the zoom motor 112 are thus interlocked in the same manner as the female helicoid 106 and the focus motor 114.
The most typical arrangement of the conventional front-lens focus type zoom lens is as described above. In the case of the front-lens focus type zoom lens, the front lens part is arranged to be drawn out accordingly as the focusing distance becomes nearer. The amount of drawing out the lens tends to increase in proportion to the reciprocal number of the distance. Therefore, the nearest shootable distance with the front-lens focus type zoom lens is about one meter in general.
Meanwhile, zoom lenses called inner focus type or rear focus type zoom lenses have been known. The zoom lens of this type is arranged to perform a focusing action by using a lens group which is disposed more rearward than the variator lens. Such zoom lenses have already been manufactured and marketed. The inner focus type zoom lens permits shooting at a nearer distance than the front-lens focus type zoom lens and is arranged to permit, on the wide-angle side thereof, continuous focusing on objects located at distances from a point immediately before the lens to an infinitely distant point.
FIG. 9 shows by way of example the arrangement of one of the various zoom lenses of the inner/focus type which uses the rearmost lens group for focusing. Referring to FIG. 9, this zoom lens consists of a fixed front lens 1; a variator lens 2 for varying the lens power; a fixed lens 3 used as a relay lens group; and a focus lens 4. A guide bar 133 serves as a whirl-stop. A reference numeral 134 denotes a variator-lens feeding bar. A numeral 135 denotes a fixed tube. A numeral 136 denotes a diaphragm unit, which is assumed to have been inserted perpendicularly to the paper surface of the drawing. A stepping motor 137 is employed as a focus motor. The stepping motor 137 has an output shaft 138 which is threaded for lens feeding. This lens-feed screw part of the output shaft 138 engages a female screw part 139 which is formed in one body with a moving frame 140 of the focus lens 4. Guide bars 141 and 142 are arranged to guide the lens moving frame 140. A back plate 143 is arranged to position and retain the guide bars 141 and 142 in place. The illustration includes: a relay holder 144; a zoom motor 145; a zoom-motor speed reduction gear unit 146; and interlocking gears 147 and 148. The gear 148 is secured to the variator-lens feeding bar 134.
With the zoom lens arranged in the above-stated manner, when the stepping motor 137 is driven, the focus lens 4 is moved by screw feeding in the direction of the optical axis. Further, when the zoom motor 145 is driven, the gears 147 and 148 move to rotate the variator-lens feeding bar 134. The rotation of the feeding bar 134 causes the variator lens 2 to move in the direction of the optical axis.
FIG. 10 shows the positional relations between the variator lens and the focus lens obtained in the above-stated zoom lens for various object distances. In this instance, the positional relations are shown by way of example as obtained with the lens focused on objects located at distances including an infinity distance, 2 m, 1 m, 80 cm and 0 cm. As shown, in the case of the inner focus type zoom lens, the positional relation between the variator lens and the focus lens varies according to the object distance. Compared with the cam ring arrangement of the front-lens focus type zoom lens, therefore, the lens groups of the inner focus type zoom lens cannot be interlocked by a simple mechanical arrangement. If the lens of the structural arrangement as shown in FIG. 9 is arranged to be simply operated by driving the zoom motor 145, the image of the object would then be blurred.
The above-stated characteristic has delayed the practicalization of the inner focus type zoom lens despite its advantages over the front-lens focus type zoom lens, including: its excellent ability in respect of nearest distance shooting; a fewer number of component lenses required; and a simpler mechanical arrangement.
During recent years, however, techniques for performing optimum control over a positional relation between the component lenses according to the object distance as shown in FIG. 10 have been developed and practicalized. For example, a method for tracing the positional relation between two lenses 2 and 4 according to the object distance has been proposed as disclosed in U.S. Pat. No. 4,920,369.
The above-stated method is as shown in FIGS. 11, 12 and 13. The positional relation between the variator lens and the focus lens can be maintained by this method. FIG. 11 shows in a block diagram an arrangement for carrying out the method. In FIG. 11, reference numerals 1 to 4 denote the same lens groups as those shown in FIG. 9. The position of the variator lens 2 is detected by a zoom encoder 149. The zoom encoder 149 may be arranged as a volume encoder, for example, to have a brush mounted on a variator-lens moving ring in one body therewith and to allow the brush to slide over a circuit board on which a resistance pattern is printed. An aperture encoder 150 is arranged to detect an aperture value by using, for example, the output of a Hall element which is disposed within an aperture meter 163. An image sensor 151 is a CCD or the like. A camera signal processing circuit 152 supplies a luminance signal Y to an AF (automatic focusing) circuit 153. The AF circuit 153 makes a discrimination between an in-focus state and a defocus state. In the event of defocus, the defocus state is determined to be a near-focus or far-focus state and the amount of defocus is also determined. The result of these processes are taken into a CPU 154. A power-on reset circuit 155 is arranged to perform reset actions when a power supply is switched on. A zoom operation circuit 156 is arranged to transmit to the CPU 154 information about an operation performed by the operator on a zoom switch 157. Memory parts store the data of loci shown in FIG. 10, including information data 158, speed data 159 and boundary data 160. A zoom motor driver 161 is arranged to drive a zoom motor 145. A stepping motor driver 162 is arranged to drive a stepping (focus) motor 137. The number of pulses inputted to the stepping motor 137 is continuously counted and supplied to the CPU 154. The stepping motor driver 162 is thus used as an encoder for the absolute position of the focus lens 4. In this arrangement, the position of the variator lens 2 and that of the focus lens 4 are thus respectively obtained through the zoom encoder 149 and the numbers of input pulses applied to the stepping motor 137. Therefore, one point can be decided on a map which is shown in FIG. 10.
Meanwhile, the map shown in FIG. 10 is divided into strip-like small areas I, II, III,--as shown in FIG. 12. In FIG. 12, hatched parts indicate areas where lens allocation is inhibited. With one point decided on the map, it is possible to determine one of the small areas to which the point belongs. In each of the speed data and the direction data, the speed and direction of rotation of the stepping motor obtained by the locus passing the center of each area are stored for every area. For example, in the case of FIG. 12, the position of the variator lens on the axis of abscissa is divided into ten zones. Assuming that the speed of the zoom motor is set at such a value as to move the variator lens between a telephoto end position and a wide-angle position in ten sec, the variator lens passes one zone in one sec when it is moved in the direction of zooming. FIG. 13 is an enlarged view of a block III shown in FIG. 12. A locus 164 passes the middle part of the block III. A locus 165 passes a lower left point. A locus 166 passes an upper right point. The angles of inclination of these loci differ somewhat from one another. However, with respect to the middle locus 164, the locus can be traced without much error, if the focus lens moves at a speed of X mm/sec.
The speed obtained as mentioned above is called an "area representative speed". In the memory for the speed data, the value of the area representative speed is stored for every one of the small areas. Further, assuming that this speed is indicated as 168, the speed of the stepping motor 137 is set by finely adjusting the representative speed to 167 or to 169 according to the result of detection made by an AF (automatic focusing) action. Further, with respect to the direction data, the rotating direction of the stepping motor varies according to the direction of zooming from a telephoto end position T to a wide-angle end position W or from the position W to the position T. Therefore, the data of signs indicating these directions are stored in the memory of the direction data.
The speed of the stepping motor 137 is thus set by correcting the area representative speed according to the result of (focus) detection made by the AF circuit. Then, by using the stepping-motor speed thus obtained, the position of the focus lens is controlled by driving the stepping motor during a zooming action. With the focus lens position controlled in this manner, it is possible to prevent any blurred state from taking place during the process of zooming, even if the zoom lens is of the inner focus type. The cam tracing method which is described above is called an electronic cam.
In another known method, the speeds 167 and 169 shown in FIG. 13 are stored in the memory in addition to the representative speed 168, and one of the three speeds is selected according to the result of distance measurement made by the AF circuit.
The method for controlling the lens position of the inner focus type zoom lens during the process of zooming has been described above in comparison with the front-lens focus type zoom lens. In the case of the description given above with reference to FIGS. 11, 12 and 13, the operation of the device is performed while the AF device is on. However, even when the AF device is off, the lens can be kept focused on one and the same object throughout a zooming action performed solely at the area representative speeds from the telephoto end position T to the wide-angle end position W or from the position W to the position T by reversely tracing the passing points after the zoom lens is first brought into an in-focus state in the telephoto end position. In another known method which is disclosed in Japanese Laid-Open Patent Application No. HEI 1-32146, points on the loci are stored beforehand, instead of storing the speed data in the manner as described in the foregoing.
However, in the case of manual focusing operation on such an inner focus type zoom lens, it is very difficult to change the position of the focus lens without rotating the motor as shown in FIG. 7. The reasons for the difficulty include among others the following points:
(1) The focus lens is disposed in the rear part of the zoom lens and is located closer to the inside of the camera. Therefore, the focus lens does not leave a sufficient interlocking space. (2) Since the moving ring is fitted by screwing directly on the output shaft of the stepping motor, it is difficult to form a slip unit. (3) The absolute position encoder fails to operate if the lens position is not changed by driving the stepping motor.
Therefore, the manual focusing operation on the inner focus type zoom lens has been performed by allowing the focus motor to rotate in one direction in accordance with a switch operation, in a manner called "power focusing". Switch arrangement used for this purpose includes two different types. In one type: Two push switches are simply arranged to designate the driving direction of the focus motor. In the other type: An operation ring is arranged, in the same manner as in the case of the conventional front-lens focus type zoom lens, to designate the direction and the speed of driving of the focus motor on the basis of the direction and speed of rotation of this operation ring.
In actuality, however, little heed is given to any action that is to be performed when a manual focusing operation is performed during the process of zooming. In other words, when a manual focusing operation is performed during the process of zooming even while the AF device is off, the focus motor is already driving the lens for tracing the locus. Besides, the speed and direction of the driving action vary according to the area. Therefore, with an instruction given by operating a manual focusing button to cause the motor to rotate either clockwise CW or counterclockwise CCW at a certain speed, if the instruction is accepted, a greatly blurred state would take place in a very short period of time. Further, in such a case, a faulty action tends to be performed to increase the blur even if a button by which the focus is to be shifted toward the infinity distance is pushed or a ring is operated toward an infinity distance position, or even if a button by which the focus is to be shifted toward the nearest distance is pushed, or a ring is operated toward the nearest distance position. To avoid such a faulty action, it has been generally inhibited to accept any manual focusing operation that is performed during the process of zooming while the AF device is off.